That is very common with these holsters. It is probably the result of a reaction to something in the leather from the tanning process with the brass that leaves the oily green stuff. If its really bad I usually just hold the holster upside down over a garbage can and use a small brush to knock the green corrosion off of the brass. I try to keep it off of the leather, if it gets on the leather it can leave a dark spot. After knocking it off I use Brasso or Flitz on a cleaning patch to clean the brass. If there is an oily circle on the leather around the brass hardware a little saddle soap will help but a dark area will remain.
If its not real bad I just brush the bulk of the green corrosion off and leave it alone, the patina on the brass just shows the age of the old holster and looks very fitting in my opinion.